Charge separation and carrier dynamics in donor-acceptor heterojunction photovoltaic systems

Abstract

Electron transfer and subsequent charge separation across donor-acceptor heterojunctions remain the most important areas of study in the field of third-generation photovoltaics. In this context, it is particularly important to unravel the dynamics of individual ultrafast processes (such as photoinduced electron transfer, carrier trapping and association, and energy transfer and relaxation), which prevail in materials and at their interfaces. In the frame of the National Center of Competence in Research "Molecular Ultrafast Science and Technology," a research instrument of the Swiss National Science Foundation, several groups active in the field of ultrafast science in Switzerland have applied a number of complementary experimental techniques and computational simulation tools to scrutinize these critical photophysical phenomena. Structural, electronic, and transport properties of the materials and the detailed mechanisms of photoinduced charge separation in dye-sensitized solar cells, conjugated polymer- and small molecule-based organic photovoltaics, and high-efficiency lead halide perovskite solar energy converters have been scrutinized. Results yielded more than thirty research articles, an overview of which is provided here.

FIG. 1.

Energetic scheme and typical architecture of various types of photovoltaic systems based on donor-acceptor heterojunctions: (a) Dye-sensitized solar cells (DSSCs), (b) Polymer OPV bulk heterojunction cells, and (c) Planar small-molecule-based OPV and perovskite solar cells. The voltage across the device ΔU is given by the energy difference separating the quasi Fermi levels for electrons (E*_F,n) and that for holes (E*_F,p): –$\mathcal{F}$·ΔU, where $\mathcal{F}$ is the Faraday constant.

FIG. 2.

Molecular structures of exemplary XY1 and SM315 push-pull organic dye-sensitizers discussed in the text. Color-code for atoms: C (dark grey), H (white), O (red), N (blue), S (yellow), Zn (orange). In the case of the SM315 molecule, the zinc porphyrin chromophore (C) is intercalated between the triarylamine donor moiety (D) and the π-conjugated bridge (π).

FIG. 3.

Ball-and-stick molecular model of N719 dye-sensitizer adsorbed on the TiO₂ anatase (101) surface. Ultrafast electron injection into the solid oxide takes place upon photoexcitation of the Ru(II) complex by a metal-to-ligand charge transfer (MLCT) transition. Provided the excess excitation energy is not too large (ΔE_ex < 1 eV, λ_pump ≥ 470 nm), a charge transfer exciton (CTE) is formed through Coulombic interaction between the positive charge (h⁺) located within the HOMO of the oxidized dye cation and the injected electron (e⁻) trapped at a neighboring, uncoordinated surface Ti(IV) site, with an e⁻-h⁺ separation distance ≥7 Å.

FIG. 4.

Electroabsorption spectra of pristine C₆₀ (black squares, 50 nm-thick film), Cy3-P cyanine dye (red triangles, 25 nm-thick film), and C₆₀ | Cy3-P planar bilayer (green circles, 30 nm | 20 nm-thick layers). The applied voltage was 2 V for all films. Absorption spectra of pristine 20 nm-thick films are also shown (right scale, solid lines). Vertical lines show both 390 and 575 nm excitation wavelengths chosen for selective excitation of C₆₀ and Cy3-P, respectively.

FIG. 5.

Cartoon representing a bulk heterojunction composed of neat PCBM phase (grey spheres), neat pBTTT phase (blue rods), and intermixed polymer:fullerene phase. (a) Photo-generated charges create a local electric field around them, which induces an electro-absorption signal in the transient absorption spectra, allowing to determine in which phase the charges are found. (b) A bulk electroabsorption response is induced by an external electric field, which is shielded by the transport of photo-generated charges to the electrodes, allowing to deduce the spatio-temporal separation of the electron-hole pair.

FIG. 6.

Valence band maximum (VBM) (blue frame) and conduction band minimum (CBM) (red frame) orbitals of CsPbI₃. Panels (a)–(c) show how the tilting of PbX₆ octahedra affects the overlap.

FIG. 7.

Variation of internal energy ΔE (blue circles and dashed line), mixing entropy contribution, −T·ΔS (red triangles and dotted line) and Helmoltz free energy ΔF=ΔE – T·ΔS (black squares and solid line) as a function of Cs⁺ content in the mixed cation Cs_xFA_(1–x)PbI₃ perovskite.

FIG. 8.

Schematic diagram of energy levels and electron transfer processes in a TiO₂ | perovskite | HTM cell. The structure of the cell unit of the MAPbI3 perovskite in its cubic form is shown in the top right illustration.

FIG. 9.

Time evolution of electron and hole populations in photoexcited MAPbI₃ perovskite in various systems: CH₃NH₃PbI₃ | TiO₂ (black); CH₃NH₃PbI₃ | Al₂O₃ (blue); spiro-MeOTAD | MAPbI₃ | TiO₂ (red); spiro-MeOTAD | CH₃NH₃PbI | Al₂O₃ (green). Thick solid lines represent bi-exponential fits of experimental points starting at t = 1 ps. A₂ represents the normalized absorbance change at 25 ps, used as a metric to compare the various samples. Inset: Charge recombination dynamics obtained from nanosecond laser flash photolysis of the same samples. Signals mainly reflect the decay of the h⁺(HTM) population. Solid lines represent stretched exponential fit of experimental data. All transient absorption signals were monitored at a probe wavelength λ = 1.4 μm following pulsed excitation at 580 nm.

FIG. 10.

Early time dynamics of neat MAPbI₃ (black), MAPbI₃ | Al₂O₃ (red), and CH₃NH₃PbI₃ | TiO₂ (green). (a) THz kinetics (λ_pump = 400 nm, I_exc = 1.7 × 10¹³ photons/cm² per pulse) normalized to 1. Inset shows the time-evolution of the electron mobility for the first 50 ps, obtained from the measured THz photoconductivity and carrier density. (b) NIR transient absorption kinetics (λ_pump = 603 nm, λ_probe = 970 nm, I_exc = 6.0 × 10¹⁴ photons cm⁻² per pulse). Inset displays the corresponding photoluminescence spectra (λ_pump = 550 nm).

FIG. 11.

Comparison of NIR transient absorption (TA) and time-resolved terahertz (THz) kinetics for neat MAPbI₃ showing that THz mobility remains constant for at least 1 nanosecond.

FIG. 12.

Cartoon illustrating the energy- and charge transfer processes occurring between the various nanostructures constituting MAPbBr₃ perovskite colloidal aggregates. Left: Energy- and/or charge transfer cascade (curved blue arrows) between q-2D nanoplatelets of increasing thicknesses and eventually a 3D bulk-like nanoparticle. Right: Energetic scheme of some examples of photophysical processes taking place in a nanoparticle aggregate: Upon photoexcitation of a thin q-2D (m = 3) nanoplatelet, interfacial hole transfer can take place to the adjacent particle (process I). Electrostatic interaction of the hole with an electron remaining on the other side of the interface yields a CT exciton (green ellipse). Subsequent electron transfer (process II) leads to the excitation of the m = 4 q-2D nanoplatelet. Energy transfer to a neighbouring nanostructure characterized by a narrower bandgap is then possible (process III). Interfacial electron transfer (process IV) finally enables the formation of a new interfacial CT excitonic species.

FIG. 13.

Time-evolution of electro-modulated differential absorption (EDA) spectra of insulated MAPbI₃ films excited at λ = 545 nm and submitted to an external electric field E₀ = 1.7 × 10⁵ V cm⁻¹. Inset: Time-dependence of the differential absorbance change recorded in the same conditions at λ_probe = 762 nm.

FIG. 14.

(a) Electro-absorption spectra of the perovskite film contained in a complete solar cell upon application of increasing voltages between both electrodes. Inset shows the quadratic dependence of the differential absorbance signal upon the applied external field intensity and direction. (b) Time-dependence of the EA differential absorbance change recorded upon application of an external electric field E₀ = 4.9 × 10⁴ V cm⁻¹ (U₀ = 3.5 V), E₀ = 3.5 × 10⁴ V cm⁻¹ (U₀ = 2.5 V), and E₀ = 1.4 × 10⁴ V cm⁻¹ (U₀ = 1.0 V) and probed at the peak of the electroabsorption signal at λ = 758 nm, energy fluence = 0.1 μJ cm⁻².

FIG. 15.

Simplified band alignment diagram for ETM | MAPbI₃ | HTM double donor-acceptor heterojunctions. Curved arrows represent the hole transfer processes taking place from the valence band maximum of the perovskite absorber to the various investigated hole-transport materials.

FIG. 16.

(a) Transient absorption spectra of mesoporous TiO₂ | MAPbI₃ and mesoporous-TiO₂ | MAPbI₃ | spiro-MeOTAD systems 1ps after excitation, as well as the corresponding difference spectrum. (b) Dynamics at 620 nm for mesoporous TiO₂ | MAPbI₃ and mesoporous TiO₂ | MAPbI₃ | P3HT.